The invention relates to an electromechanically actuatable disk brake with a housing and a friction pad which is slidably supported at the housing, against which an electrically actuatable actuator acts, with the actuator comprising an electric motor and a spindle/nut (transmission) arrangement.
Such vehicle brake arrangements, in particular in the form of disk brakes, are known from so-called brake-by-wire systems.
These arrangements are based on the problem of transforming the rotational movement of the electric motor into a translatory movement of the friction pads relative to the brake disk, with high feed forces being required on the one hand and high dynamics of the feed motion being required, on the other hand. In addition, there is the requirement for a high reliability with a high mechanical efficiency within a wide temperature range as occurs in vehicle brakes. Moreover, the requirement for a self-releasing capability of the friction pads from the brake disk upon a current failure of the electric motor must be met. In addition, the limited installation space at the vehicle wheel is to be utilised efficiently, and the unsprung masses are to be kept small. Another aspect with the employment of spindle/nut arrangements for the transmission of the rotational movement into a translation movement is that, if possible, no transverse forces or bending moments are introduced into the spindle transmission, because the spindles are generally not designed for this and the spindle efficiency will then considerably deteriorate.
The selection of the spindle pitch and the spindle diameter is decisively dependent on the required spindle axial force and the available drive moment at the spindle nut. In addition, the axial force provided by the spindle must be supported by an adequately dimensioned axial bearing.
DE 195 11 287 A1 shows a floating caliper brake wherein an electric motor which is formed as an outside rotor drives a nut which is rigidly coupled with the rotor of the electric motor. A screw spindle is displaced in the axial direction via threaded rollers which are in engagement with the nut, when the electric motor sets the nut and thus the threaded rollers in rotation. The screw spindle acts on a friction pad. The screw spindle is supported in the rotor of the electric motor by the threaded rollers. The rotor of the electric motor with its end facing the friction pad is guided via a radial bearing which bears against the housing.
From DE 195 43 098 A1 a floating caliper brake is known, wherein an electric motor which is formed as an inside rotor drives a screw spindle. The spindle has a male thread which is in engagement with threaded rollers, which are arranged along the circumference of the spindle in a distributed manner. A female thread of a threaded bushing engages at the periphery of the rollers. The threaded bushing is connected with a friction pad so that the friction pad is displaced in the axial direction upon a rotation of the motor. The rotor of the electric motor is provided with a hollow axle for accommodating the spindle rod. At its rear side, the rotor bears against the housing via a radial bearing and against the spindle rod via an axial bearing.
In the two above mentioned arrangements, either transverse forces of the electric motor are acting on the point of contact between the spindle and the rollers, or the friction pads generate reactive forces onto the points of contact between the spindle and the rollers under operating conditions. In both cases the consequence is increased wear and higher friction forces between the points of contact at the spindle or the rollers, respectively. The result of this is that the self-releasing capability and the precision of the actuating movement is affected.
From DE 196 05 988 A1 a floating caliper brake is known, wherein an electric motor with an inside rotor drives a screw spindle. In order to be able to manage with a small drive motor, the rotor is designed as a cup inside of which the screw spindle is arranged. The screw spindle bears with only one end against the bottom of the cup which is supported at the housing by an axial and two radial bearings. Along its longitudinal extension the screw spindle is not supported further so that the other end of the screw spindle is free. Planetary rollers are abutting the screw spindle, by means of which a rotational movement of the screw spindle is transferred into an axial displacement of a nut. The nut bears radially against the inside of the cup-shaped rotor or its radial bearings, respectively. Due to the fact that the second axial end of the screw spindle is not radially supported, this arrangement is disadvantageous in that a shift of the free end of the screw spindle, which is caused by mechanical stress and wear, results in an eccentric rotational movement so that the planetary rollers are stressed unevenly and the actuation movement becomes unprecise.
The invention is based on the object to improve a vehicle brake of the initially mentioned type in such a manner that these drawbacks are eliminated.
The inventive solution provides for the electric motor to be configured as an internal rotor motor with a stator and a rotor, the rotor of which is coupled secured against rotation with the spindle, the spindle is operatively coupled with the nut in order to transform a rotational movement of the spindle into a translatory movement of the nut, the spindle is radially supported along its longitudinal extent in at least two places so as to be rotatable relative to the housing, and the nut is guided axially slidable relative to the housing.
The bearing arrangements provided in the state of the art do not represent a solution of this problem. By means of the axial sliding guidance of the nut, the inventive configuration compensates transverse forces acting on same from the brake shoes, so that no transverse forces are transferred to the rollers. The double bearing of the spindle has the effect that transverse forces from the electric motor can be taken up by the housing, without being able to cause deformations of the spindle.
In a preferred embodiment of the invention, the spindle is in a driving connection with the nut via planetary rollers.
The spindle is rotatably supported in the area of its two ends. A first bearing is preferably designed as a locating bearing and a second bearing as a floating bearing relative to the housing in the axial direction. This permits a compensation of forces which cause an axial deflection of the spindle.
The nut is slidably guided in a sliding bushing or directly in the housing. A direct guidance of the nut in the housing (without a separate sliding bushing) reduces the manufacturing expenditure.
In the area of the end of the spindle which faces the friction pad a bearing shield is provided according to the invention, which forms a bearing position for the spindle and/or a bearing position for the nut. The bearing shield can either be formed integrally with the housing, or can be formed as a component which is separately inserted into a corresponding opening in the housing. Thus, the bearing shield can assume a twofold function with a defined correlation of the bearing position for the spindle and the bearing position for the nut being able to be manufactured precisely in a simple manner.
In a particularly preferred embodiment, the nut comprises at least one extension which can protrude through a correspondingly formed opening in the bearing shield, in order to transfer the axial movement of the nut through the bearing shield to the outside to the friction pad.
The second bearing for the spindle (i.e. the floating bearing) is preferably accommodated in a corresponding recess in the bearing shield and bears against the housing via webs which are arranged between the opening between the extensions of the nut. This particularly preferred embodiment permits a very compact and closed configuration of the arrangement.
The extension or each extension of the nut as well as the respective associated opening are preferably designed in such a manner that the nut is secured against rotation relative to the housing. In this manner, the nut is additionally guided both in the axial and in the radial direction towards the area of the openings.
In order to avoid the ingress of dust or humidity into the interior of the housing, an intermediate element is preferably arranged between the friction pad and the extension or the extensions of the nut, which serves as an end piece of the extension(s) of the nut or for accommodating a seal (e.g. in the form of a bellows).
In a particularly preferred manner the end of the spindle, which is accommodated at the floating bearing, is biased by a spring arrangement in the axial direction against the housing.
This can compensate for expansions of the spindle or the housing, but also of other components, which are primarily caused by temperature variations.
In a further configuration the spindle is coupled with a spring-loaded cylinder which upon a feed motion takes up work which is directed opposite said feed motion and which causes a resetting movement of the spindle or the nut, respectively, with a currentless electric motor. In this manner, it is ensured that the friction pad clears the brake disk.
In order to achieve a particularly space-saving arrangement of the spring-loaded cylinder this can be designed as a torsion rod formed in a longitudinal hole of the spindle. This arrangement is advantageous in that the spindle/nut arrangement can be designed self-locking so that a parking brake function can be realised. By a controlled charging or discharging, respectively, of the spring-loaded cylinder, the spindle can be brought into rotation by the spring-loaded cylinder against the feed force, even with a non-functional electric motor, so that the brake disk and the friction pad clear each other.
The spring-loaded cylinder is preferably arranged relative to the spindle, the motor, and the housing in such a manner that the charging/discharging process of the spring-loaded cylinder can be controlled by means of a coupling which is coupled with the spring-loaded cylinder.
It is particularly preferred that the disk brake comprises a floating caliper which carries two friction pads between which the brake disk is arranged. The floating caliper is designed in such a manner that, upon a feed motion, it takes up work which is directed against said feed motion and which causes a resetting movement of the spindle or the nut, respectively, with a currentless electric motor so that the friction pad clears the brake disk. This configuration permits the omission of a special spring-loaded cylinder and forms a particularly simple construction of the disk brake.
In a further embodiment, a further transmission stage, preferably in the form of a planetary transmission, is provided in addition to the roller/spindle or nut/spindle (transmission) arrangement.
Further characteristics, properties, and modification possibilities will be explained by means of the description of three embodiments with reference to the accompanying drawings.